The present invention relates to a method for purifying anhydrides, particularly aromatic polyanhydrides such as bis phenyl furanediones, to form novel metal-free polyanhydrides suitable for reaction with other metal-free monomers to form metal-free polymers having excellent electro-resistive properties.
Aromatic polyanhydrides generally are produced by the liquid phase air oxidation or nitric acid oxidation of the corresponding alkyl aromatics in the presence of a metal salt catalyst. Such commercially-available polyanhydrides commonly contain from 100 to 500 parts per million of such metal salts in the form of impurities. For example crude commercially-available 2,2-bis-(3,4-dicarboxy phenyl) hexafluoropropane dianhydride (sometimes referred to as 5,5-[2,2,2-trifluoro-1-(trifluoromethyl) ethylidene]bis 1,3-isobenzofuranedione) (6F-DA) generally contains about 80 ppm sodium, 20 ppm iron, 30 ppm cobalt and 30 ppm manganese, for a total of about 160 ppm of these impurities. Prior known methods for the purification of crude commercially-available anhydrides result in partially purified anhydrides which still have an unduly high content of residual trace metals i.e., up to about 50 ppm of sodium, 10 ppm iron, 10 ppm cobalt and 10 ppm manganese, unless the purification procedure is repeated one or more times in order to remove the impurities in increments. Such incompletely purified polyanhydrides are not suitable for the formation of electronic grade polymers because the presence of such high amounts of trace metals in the formed polymers renders the polymers sufficiently electro-conductive that they are unsuitable for use in electronic components requiring electro-resistive properties. For example, polyimides used for electronic applications are required to be essentially free of metal ions and therefore must be produced by the reaction of substantially metal-free monomers or reactants, such as dianhydrides and diamines. A preferred class of polyimides comprises the high temperature-resistant fluorinated polyimides formed by the reaction of fluorinated aromatic polyanhydrides such as 6F-DA or 12F-DA with fluorinated or unfluorinated polyamines, such as 2,2-bis (3 aminophenyl) hexafluoropropane or 4,4'diamino-phenyl ether.
One method for purifying and clarifying aromatic polyanhydrides comprises dissolving the polyanhydride in acetone, adding absorbent such as carbon to clarify the solution, filtering, distilling off the acetone, adding toluene to crystallize out the purified polyacid, and adding acetic anhydride to convert any formed polyacid back to the cyclic polyanhydride. Generally the polyanhydride is initially formed in the presence of a metal salt catalyst by cyclization of a polyacid and this anhydride contains substantial trace amounts of metal from the catalyst and/or from other sources which cannot be completely removed in a single operation by washing, carbon desorption or other conventional purification systems. Repeated purification procedures do remove substantial additional amounts of the trace impurities, to about the same extent as the present purification process, but the repetition of the purification steps is time-consuming and otherwise disadvantageous.
Reference is made to U.S. Pat. Nos. 2,985,665; 2,518,312; 3,529,017; 3,544,602 and 3,965,125 for their disclosures of prior known processes for producing and/or purifying organic anhydrides.
Reference is also made to U.S. Pat. Nos. 4,592,925 and 3,959,350 for their disclosure of processes for the formation of polyimides from aromatic polyanhydrides and polyamines.